Metal nanowires are of particularly advantageous application in the manufacture of transparent electrically conductive materials, in particular of transparent electrodes, in particular for optoelectronic devices (photovoltaic devices, OPDs (Organic Photo Detectors), screens, etc.), or transparent heating films.
Recent advances in the nanotechnology field have shown that metal nanowires, for example silver nanowires, constitute a particularly advantageous alternative to films based on transparent conductive oxides (known under the abbreviation “TCO”), for example based on indium tin oxide, conventionally used for producing transparent electrodes.
The transparent conductive system is then obtained by forming, from a suspension of nanowires in a solvent (for example, water, methanol, isopropanol, etc.), a percolating network of metal nanowires on a surface, for example made of glass. Numerous advantages are associated with this manufacturing process: low cost, flexibility of the electrodes obtained, wet-processability at low temperature, etc., as described in the publication Langley et al. [1].
Metal nanowires are generally produced, easily and in large amounts, by chemical synthesis in solution, via the reduction of metal salts, for example of silver nitrate for obtaining silver nanowires, with a polyol, generally ethylene glycol.
Unfortunately, this synthesis is not a selective reaction, and impurities are produced during synthesis, in particular small nanoparticles, aggregates (agglomerated nanoparticles) and large particles, for example of rod type. In point of fact, these coproducts are capable of having a negative impact on the performance levels of the electrodes (drop in transmittance at identical conductivity, substantial roughness, etc.).
Consequently, it is advisable to remove these various by-products of the nanowire synthesis reaction, in order to optimize the performance levels of the electrodes and to facilitate their integration, in particular into structures comprising other layers in direct interaction with the electrodes.
Unfortunately, the nanowire purification technique commonly used, which consists of a succession of centrifugation steps, is lengthy, fastidious and difficult to reproduce.
Alternative methods for purifying nanowires have already been proposed. For example, application US 2011/0045272 describes a process for isolating metal nanowires, by precipitation of nanostructures having an aspect ratio greater than 10, from a ternary mixture formed by combining the reaction mixture comprising the nanowires in suspension in a polyol solvent, with water and a ketone.
Mention may also be made of Mayousse et al. [2] who propose leaving the reaction mixture after silver nanowire synthesis to settle out for several days. This settling out makes it possible to separate the nanoparticles which are in the supernatent phase, from the nanowires present in the precipitate. While this process makes it possible to be rid of the nanoparticles, the aggregates and other particles of large size are, together with the nanowires, in the precipitate, and are not therefore separate from the metal nanowires.